I say to you, that every careless word that men shall speak, they shall render account for it in the day of judgment. For by your words you shall be justified, and by your words you shall be condemned. (Matthew 12:36-37).
If anyone does not stumble in what he says, he is a perfect man, able to bridle the whole body as well…The tongue is a small part of the body, and yet it boasts of great things. Behold, how great a forest is set aflame by such a small fire! And the tongue is a fire, the very world of iniquity; the tongue is set among our members as that which defiles the entire body, and sets on fire the course of our life, and is set on fire by hell…It is a restless evil and full of deadly poison. (James 3:2,5-6,8).
If anyone considers himself religious and does not keep a tight reign on his tongue, he deceives himself and his religion is worthless. (James 1:26, NIV).
The one who guards his mouth preserves his life; the one who opens wide his lips comes to ruin. (Proverbs 13:3).
Set a guard, O LORD, over my mouth; keep watch over the door of my lips. (Psalm 141:3).
See also Psa 34:11-14, Prov 10:19, 21:23.